A STUDY OF ATMOSPHERIC PARTICLES

Abstract

The thesis entitled "A Study of Atmospheric Particles" deals with properties of the atmospheric particles, especially the optical properties, which have been investigated for particles of different origin and size distribution. The minute suspended particles in the atmosphere, the aerosols, exert a profound influence on the optical properties of atmosphere. The particles scatter incident radiation in different directions in varying proportions and affect angular distribution of diffused radiation. Aerosols exert a direct effect on climate primarily through their interaction with solar radiation and, to a lesser degree, through their interaction with infrared radiation. Non-absorbing aerosols will scatter more sunlight back into space, thus reducing the amount of solar radiation reaching the surface. Aerosols, which absorb solar radiation, cause a warming of the absorbing atmospheric layer and a cooling of the underlying surface. As the relative humidity increases, water vapour condenses out of the atmosphere onto the particulates suspended in the atmosphere. This condensed water increases the size of the aerosols and changes their composition and their effective refractive index. The resulting effect of the aerosols on the absorption and scattering of light will correspondingly be modified. Anthropogenic sources have increased significantly over past century mainly due to fossil fuel use and biomass burning. The principal contributors to aerosol radiative forcing produced from these anthropogenic sources are believed to be black carbon and sulphate particles. Whereas sulphate particles mainly scatter solar radiation, black carbon particles scatter and absorb and may therefore reduce the cooling by sulphate aerosols, at least over regions with high surface albedo. Recently much attention has been given to the (i) behaviour of aerosols because of their increasing abundance in the atmosphere. The present work aims at investigating various optical properties of atmosphere particles and their implications. For a systematic presentation of the whole work, the thesis has been divided into five chapters. The first chapter gives an introduction to the subject of the thesis and basic idea of the work to be presented in the other chapters. The work already done in this field is briefly reviewed and recent theoretical and experimental development related to the present work is given. Apart from that a brief introduction of aerosols origin in the atmosphere, their properties like shape, size, number density, size distribution functions, chemical composition etc. are presented. The various methods for sampling the aerosols and measuring the scattering properties are also discussed. The theories of elastic scattering of electromagnetic radiation by particles, Rayleigh and Mie theory are reviewed in detail as they are used in computing the optical properties. Propagation of electromagnetic radiation at optical, infrared wavelength through the atmosphere is affected by absorption by haze particles or aerosols becomes the dominant factor in the boundary layer near the earth's surface, especially under low visibility conditions. It would be interesting to study the effect of an increase in relative humidity on angular scattering of light by aerosols (by incorporating the changes due to it as the complex refractive index and the size distribution functions). Therefore, in second chapter, the properties of atmospheric aerosol particles, as functions of the relative humidity at thermodynamic equilibrium with the surrounding moist air have been investigated. Calculations have been done using urban haze and maritime aerosol models. The particles follow the power law size frequency distribution. The size frequency distribution shifts toward larger equivalent radius with increasing relative humidity. The degree of polarization of the scattered intensity is computed for different values of scattering angle by varying the size of the particle. The dependence of scattering and absorption efficiencies and asymmetry factor on relative humidity for particles of different sizes has been studied. The variation in degree of polarization for smaller particles 4, is systematic, whereas, for bigger particles it is not so. The efficiency factor for scattering does not show any significant change with humidity for bigger particles but it reveals an increase with humidity after 60% for smaller particles. This kind of study is useful in understanding the atmospheric visibility in hazy/ foggy environment. Chapter third deals with the studies on measurements of ambient aerosol concentration using particle counter. The particle counter monitors the number concentration in four different size ranges viz. 0.3-0.5 pm, 0.5-1 pm, 1-2 pm, and 2-5 pm respectively. The concentration of airborne particles is measured by exposing the particles to light and counting the number of particles by size. Observations have been taken to study the seasonal variation of aerosol number concentration and their dependence on atmospheric parameters like relative humidity, air temperature, rainfall, and wind speed. Measurements were made to examine the effect of total solar eclipse (24 October 1995) on the atmospheric aerosols. The observations were taken on the day of the eclipse (24 Oct.) as well as on the preceding (23 Oct.) and the succeeding days (25 Oct.). The results reveal an increase in aerosol number concentration in the sub-micron and micron size ranges. The increase occurred after a time lag of. about 70 minutes from the beginning of the eclipse. These results can be explained in terms of the effect of condensational growth due to increased relative humidity. When only two substances, in chemical equilibrium with each other, are responsible for all absorption in a wavelength region, there must exist at least one wavelength in the spectrum of this region where the absorption coefficient will be independent of the ratio of the concentration of the two substances. Chapter four deals with the computation of extinction spectra for aqueous solutions of sulphuric acid, nitric acid and hydrochloric acid, which can be found as fine droplet aerosols in the atmosphere. If the complex index of refraction for a liquid aerosol is known as a function of wavelength, then the extinction coefficients can be computed from the Mie theory. In these extinction spectra of liquid droplet aerosols of sizes ranging from 0.1 flril to 10 1.1111, it has been found that there exists one wavelength where the extinction coefficient is independent of the ratio of the concentrations of the two substances. Such points can serve as reference wavelength for aerosol spectrometry and also for estimation of liquid content in haze, fog, and non-raining clouds. Black carbon and inorganic sulphates and nitrates frequently constitute a major fraction of the ambient aerosol. In order to make reliable estimates of the effect of these particles on the global radiation budget, it is necessary to know their optical properties for the full spectrum of wavelengths. The radiative forcing due to aerosols is highly dependent upon the optical properties of the aerosol. Therefore, in chapter five, the wavelength dependent optical properties of black carbon, sulphates and nitrates aerosols are determined from their complex refractive indices and characteristic particle size distributions. The extinction coefficients are computed for black carbon and sulphates particles with a lognormal size distribution by use of an aerosol model (Global Aerosol Data Set). The effect of change in wavelength on extinction coefficients for a wide range of lognormal particle size distribution (defined by geometric mean radius and geometric standard deviation) has been studied. Wavelengths from (iv) the visible through the middle it were considered. In particular, atmospheric sulphates are often found to be associated with the fine particulate mass, in the size range between 0.1 and 1 where a particle scatters sunlight most efficiently. Thus, a strong correlation between visibility degradation and sulphate mass concentration has always been observed. (